A linear array of diode-lasers (edge-emitting semiconductor lasers) bar usually includes a plurality of individual diode-lasers (emitters) distributed along a substrate or “bar” comprising a plurality of semiconductor layers epitaxially grown on an electrically conductive semiconductor substrate. This type of array is usually termed a “diode-laser bar” by practitioners of the art. Such a bar usually has a length of about 10 millimeters (mm), a width of between about 1 mm and 1.5 mm, and a thickness of between about 100 micrometers (μm) and 300 μm. The emitters (diode-lasers) of the bar are formed in the epitaxial layers.
In a diode-laser bar configured to deliver near infrared radiation with a power of about 1 Watt (W) per emitter or more, the width of the emitters is typically between about 50 μm and 200 μm. Usually, the wider the emitter the higher the power output of an individual emitter. The number of emitters in a bar is determined by the length of the bar, the width of the emitters, and the spacing therebetween. Twenty emitters per bar is not an uncommon number of emitters per bar.
A diode-laser bar is usually packaged by bonding the bar in thermal contact with a heat-sink. A common application of a diode-laser bar is to provide optical pump-radiation for a energizing a solid-state laser gain-medium. The wavelength of light (radiation) emitted by the bar is usually required to correspond with a particular absorption peak of the gain medium. By way of example, neodymium-doped yttrium orthovanadate (Nd:YVO4), which is commonly used as a gain-medium in solid-state lasers having an output wavelength of 1064 nanometers (nm), has an absorption peak at 808 nm. This absorption peak has a FWHM bandwidth less than 2.0 nm. Ideally, the diode-laser bar output in steady state operation should have a peak wavelength equal to the absorption peak wavelength and a comparable FWHM bandwidth.
The growth process for diode-lasers in a bar is such that it is difficult to produce a batch of diode-laser bars each having exactly the same emitting wavelength, let alone a particular emitting wavelength. Usually such a batch of bars must be sorted to isolate those bars that have the desired emitting wavelength. Further, in any bar having nominally a desired emitting wavelength there may be a distribution of individual emitter wavelengths about that nominal emitting wavelength. The wider this distribution, the wider will be the FWHM bandwidth of the combined output of the emitters. There is a need for a passive tuning method for diode-laser bars that will at least allow minor adjustments to be made to the combined emitting wavelength for improving batch yield. Preferably, the tuning method should allow for differentially tuning emitters in a bar to narrow (or broaden) the combined emitting bandwidth of the bar.